1. Field of the Invention
The present invention relates to a method of and an optical amplifier for amplifying wavelength division multiplexed (WDM) signal light including a plurality of signal channels of different wavelengths, and in particular relates to an optical amplification technique for reducing a deviation in optical output level that occurs between signal channels of each wavelength, due to variations in optical input level.
2. Description of the Related Art
A WDM optical transmission system is operated with signal channels according to the demand for traffic transmission capacity. Usually, the number of signal channels at the start of operation is set to be comparatively few, and the signal channels are increased according to an increase in demand, to expand the transmission capacity.
The optical amplifier (hereafter, also referred to as WDM optical amplifier) used for the above mentioned WDM optical transmission system generally must have the following basic capabilities (1) to (3):    (1) Low output level deviation (flat gain wavelength characteristic)    (2) Wide input dynamic range capable of corresponding to differences in transmission distance or loss on the transmission path    (3) Low noise figure (NF) characteristic.
Conventionally, a WDM optical amplifier is designed to have these basic capabilities, and so as to acquire the required optical output power when the number of input signal channels is a maximum. Specifically, as shown for example in FIG. 15, a configuration is known in which a variable optical attenuator (VOA) 103 is connected between the stages of a two-stage configuration of optical amplifier sections 101 and 102 (for example, erbium doped fiber amplifiers (EDFA) or the like) which are capable of collectively amplifying signal channels of all wavelengths, and the attenuation of the variable optical attenuator 103 is adjusted so that the total power of the WDM signal light output from the post-stage optical amplifier section 102 becomes constant.
Moreover, as shown in FIG. 16, a configuration has also been proposed in which a WDM signal light input to a WDM optical amplifier is demultiplexed into a plurality of wavelength bandwidths using a demultiplexer 201, and signal light of each wavelength bandwidth is amplified in corresponding optical amplifier sections 2021 to 202N, and then the optical output power is adjusted in variable optical attenuators (VOA) 2031 to 203N, and each signal light output from each of the variable optical attenuators 2031 to 203N is multiplexed using a multiplexer 204 (for example, see Japanese Unexamined Patent Publication No. 2002-57389, and Japanese Unexamined Patent Publication No. 2001-53686).
Incidentally, in a conventional WDM optical amplifier such as shown in FIG. 15, since the pumping light power required in the optical amplifier sections 101 and 102 increases as the number of signal channels increases, it is necessary to mount a pumping light source of large output capacity in order to collectively amplify all signal channels. However, since a pumping light source of large output capacity is expensive, there is a drawback in that a WDM optical amplifier equipped with such a pumping light source becomes a high-cost product.
Moreover, the aforementioned WDM optical amplifiers as shown in FIG. 15 and FIG. 16 are usually also used in an operating condition where the number of signal channels in the WDM optical transmission system are few. Therefore, in a WDM optical transmission system that performs multiple repeatedly transmission using a conventional WDM optical amplifier, in the case where operation with few signal channels at the time of the initial implementation is assumed, then as shown in FIG. 17, this becomes a product for which the equipment cost per number of operation channels is comparatively high. As a result, there is a problem in that for a client who purchases the WDM optical transmission system, in a system that uses a conventional WDM optical amplifier cost at the time of initial implementation is high, and capitalization is not cost effective in an operating condition where transmission capacity is low. In addition, if the expected maximum transmission capacity is predicted incorrectly at the time of initial implementation, there is a possibility of having an operating condition where the maximum number of transmittable signal channels is not reached, and the client carries the risk of excessive capital investment. In the future, as densification technology for WDM signal light advances, the total output power of the WDM optical amplifier will become greater requiring considerable pumping light power. Therefore the risk mentioned above is expected to become more prominent.
Furthermore, in relation to the aforementioned configuration of the conventional WDM optical amplifier as shown in FIG. 16, there has been a problem in that the variable optical attenuators 2031 to 203N are provided individually corresponding to each wavelength bandwidth in order to make the signal light power of the respective wavelength bands which have been demultiplexed in the demultiplexer 201 and respectively amplified in each of the optical amplifier sections 2021 to 202N, constant at a desired level to acquire a flat wavelength characteristic. Therefore the configuration becomes complex which causes an increase in cost. In relation to this problem, for example, instead of providing each of the variable optical attenuators 2031 to 203N, it is also considered to adjust the gain of each of the optical amplifier sections 2021 to 202N so as to control the signal light power of each bandwidth to become a constant level. However, in this case, if constant output control is performed in relation to fluctuations in the total power of the WDM signal light input to the WDM optical amplifier (input dynamic range) by adjusting the gain of each of the optical amplifier sections 2021 to 202N, the gain wavelength characteristic of each of the optical amplifier sections 2021 to 202N respectively changes. Hence the deviation in optical output level over the entire wavelength bandwidth of the WDM signal light becomes greater. Therefore, there is a problem in that it becomes difficult to satisfy fully the aforementioned basic capabilities (1) and (2).